Continuous separation processes for the selective adsorption of para-xylene from a mixture of other xylene isomers, ethylbenzene, and non-aromatic hydrocarbons are common in industry. Generally, the processes use a solid adsorbent which preferentially retains the para-xylene in order to separate the para-xylene from the rest of the mixture. Often, the solid adsorbent is in the form of a simulated moving bed, where the bed of solid adsorbent is held stationary, and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds. The shift in the locations of liquid input and output in the direction of the fluid flow through the bed simulates the movement of the solid adsorbent in the opposite direction. In one commercial embodiment of a simulated moving bed adsorption apparatus, the Parex™ process, moving the locations of liquid input and output is accomplished by a fluid directing device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time, the rotary valve advances one index and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to a new valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time is uniform for each valve step in a valve cycle, and is generally from about 60 to about 120 seconds, such as 90 seconds. A typical process contains 24 adsorbent sub-beds, 24 distributors located between the 24 adsorbent sub-beds, two liquid input lines, two liquid output lines, and associated flush lines.
The principal liquid inputs and outputs of the adsorbent system consist of four streams: the feed, the extract, the raffinate, and the desorbent. Each stream flows into or out of the adsorbent system at a particular flow rate, and each flow rate is independently controlled. With reference to FIG. 1, the feed, which is introduced to the adsorbent system 100 through fluid communication conduit 105, contains the para-xylene (PX), represented as P in the composition chart, which is to be separated from other components in the feed stream, which typically include ethylbenzene (EB), metaxylene (MX), orthoxylene (OX), toluene, various C9+ aromatics, and non-aromatics, collectively represented as C in the composition chart. The desorbent, represented as D in the composition chart, which contains a liquid capable of displacing feed components from the adsorbent, is introduced to the adsorbent system 100 by fluid communication conduit 101. The extract, which is withdrawn from the adsorbent system 100 by fluid communication conduit 103, contains the separated para-xylene which was selectively adsorbed by the adsorbent and desorbent liquid. The raffinate, which is withdrawn from the adsorbent system 100 by fluid communication conduit 107, contains the other xylene isomers, ethylbenzene, non-aromatic hydrocarbons which were less selectively absorbed by the adsorbent and desorbent liquid. There also may be associated flush streams introduced to and withdrawn from the adsorbent system. These flush streams can vary in composition and rate and can include but are not limited to para-xylene, ethylbenzene, metaxylene, orthoxylene, and desorbent. The flush flow rates are typically independently controlled. The four principal streams are spaced strategically throughout the adsorbent system and divide the sub-beds into four major zones, each of which performs a different function.
Referring to FIG. 1, Zone I contains the adsorbent sub-beds located between the feed input and the raffinate output, and selective adsorption of the para-xylene takes place in this zone. Zone II contains the adsorbent sub-beds located between the extract output and the feed input, and the desorption of components other than para-xylene takes place in this zone. Zone III contains the adsorbent sub-beds located between the desorbent input and the extract output, and the para-xylene is desorbed in this zone. Finally, Zone IV contains the adsorbent sub-beds located between the raffinate output and the desorbent input, and the purpose of this zone is to prevent the contamination of the para-xylene with other components. The flush flows are introduced in the sub-beds of some of the major zones and create minor zones which are a function of the major zone rates and the smaller flush flow rates. The graph in FIG. 1 is a fluid composition chart showing the dynamics of the relative percentages of the three components (P-para-xylene, C-other feed components, D-desorbent) in fluid moving through the system. The composition of the fluid within the system is illustrated on a corresponding reference between system 100 and the graph. As the locations at which the various streams enter and leave the system change, the zones and corresponding composition profile move as well.
Two other important streams are the pumparound and pusharound streams. In a typical para-xylene separation process, the adsorbent bed consisting of 24 sub-beds is split into two main chambers. One chamber contains sub-beds 1 through 12 and the other contains sub-beds 13 through 24. Although functionally the adsorbent system as a whole does not have a top or a bottom, each chamber has a physical top and bottom. The pumparound and pusharound streams each conduct the liquid effluent exiting the physical bottom of one adsorbent bed chamber back up to reenter the physical top of the other adsorbent bed chamber. The pumparound stream is the stream that conducts the effluent of sub-bed 24 from the physical bottom of the second chamber to reenter sub-bed 1 at the physical top of the first chamber. The pusharound stream conducts the effluent of sub-bed 12 from the physical bottom of the first chamber to reenter sub-bed 13 at the physical top of the second chamber. It is important to note that the composition of the pumparound or pusharound stream changes with each valve step, and in one valve cycle, both streams will have sequentially carried the composition which corresponds to each valve position.
In refineries that produce large volumes of xylenes or have feedstreams with different concentrations of para-xylene, i.e., reformate, transalkylation, toluene disproportionation, selective toluene disproportionation, xylene isomerization, filtrate from a p-xylene crystallizer, and so on, a simulated moving bed process with dual rotary valves for additional feed and/or flush streams may be used. Operating the dual rotary valves independently allows for optimization of multiple feed locations and/or an increased number of flushes. Such a process is described in U.S. Pat. No. 8,168,845. However, there is an ongoing need to optimize the efficiency of the simulated moving bed process.